Switch mode power supplies are used in a wide variety of household and industrial appliances that require a regulated direct current (DC) voltage for their operation. There are a wide variety of known DC-DC converter topologies using PWM (pulse width modulation) or PFM (pulse frequency modulation) control to regulate output voltage.
One type of DC-DC converter topology is a resonant switched mode power converter. A resonant converter included in a resonant switched mode power converter controller with PFM utilizes resonant properties to provide natural soft switching in a closed loop to regulate the output. A resonant converter using PFM senses the power supply output and controls it by varying the switching frequency. An advantage of a resonant converter with PFM is that with the natural soft switching, there is reduced switching loss compared to non-resonant power converter topologies. Another advantage is that resonant converter with PFM can be designed to operate with higher frequencies in a smaller package sizes.
Among a variety of resonant switched mode power converters are high frequency (HF) transformer isolated LLC converters, which have become increasingly popular in recent years. LLC resonant converters utilize the resonance between two inductors and a capacitor to provide natural soft switching. LLC resonant converters save on cost and size by utilizing the magnetizing and leakage inductance of the HF transformer as part of their resonance components. One disadvantage of some resonant converters is that the required wide range of frequency control result in more complicated electromagnetic interference (EMI) filter designs. However, with the gain characteristics of LLC resonant converters, output regulation can be achieved with a narrow band of frequency control.
To interface with low frequency (e.g. 60 or 50 Hz) alternating current (AC) networks, DC-DC LLC resonant converters require an AC-DC front stage. With continuing efforts to conserve energy, there are increasingly tighter obligatory standards for the overall efficiency, low load/no load and standby power consumption in power converters.
One known method for reducing power loss at low load and no load conditions in LLC resonant converters is a mode of operation commonly known as skip or burst mode, which involves ceasing the switching activity for periods of time during the low load and no load conditions. Essentially, switching of the power switches is ceased for periods, allowing the output voltage to fall slightly, and then switching is momentarily resumed to refresh the output voltage in a “burst.” Thus, there are periodic or occasional bursts of switching activity during low or no load conditions. However, known LLC resonant converters having a skip or burst mode simply cease switching and resume switching without any consideration for the state of the primary circuit. Given that the resonant capacitor can be at a random state of charge, simply resuming switching operation can result in high current transients through the switches, rather than the typical soft switching of normal operation for resonant converters.
Accordingly, there is a need for a method and apparatus for reducing switching losses when resuming switch operation in skip mode under light load conditions in a resonant converter.
Those skilled in the field of the present disclosure will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Well known elements, structure, or processes that would be necessary to practice the invention, and that would be well known to those of skill in the art, are not necessarily shown and should be assumed to be present unless otherwise indicated. Common terms are carried forward throughout the following description, and refer to substantially similar subject matter, even if not commonly numbered for reference in particular drawings.